One of the limitations to the realization of very dense semiconductor integrated circuit arises from the fact that aluminum and aluminum alloy electrical conductors, which are required to carry high currents, develop open circuit conditions during use even after testing. These open circuit problems have been attributed to electromigration-induced failures as is now well understood in the art. Electromigration damage has been found to be held to tolerable limits if conductor widths are held to seven microns or greater for current density levels required by practical devices. But such a limitation constrains the effort to reach higher and higher packing densities.
Conductor width dependence of electromigration life in Al-Cu, Al-Cu-Si, and Ag conductors is discussed in an article, so entitled by G. A. Scoggan, B. N. Agarwala, P. P. Peressini, and A. Brouillard appearing on page 151 in The Proceedings of the 13th Reliability Physics Symposium, IEEE, New York, 1975, page 151. That reference describes the effect of varying grain size and line width on lifetime and illustrates that as the conductor line width is decreased the electromigration lifetime approaches a saturation level or even exhibits a weak minimum. Thus it concludes that large grain size leads to improved lifetimes for short, narrow lines. On the other hand, it is clear from the publication that lines even as narrow as one micron would contain structural defects or divergent sites causing electromigration failures. Consequently, even lines with large grain size are limited in their usable lengths. The above-mentioned publication notes also the desirability of an ideal bamboo or chain geometry for the grain structure in electrical conductors. The Journal of Applied Physics, Vol. 41, No. 10, Sept. 1970, at page 3954 et seq. documents the decreasing electromigration life with increasing length. Therefore, the combination of long lines (&gt;1 cm) with narrow widths presents a formidable obstacle to the use of fine-line Al.
The present invention is directed at the problem of achieving fine-line width conductor patterns with markedly improved resistance to electromigration related open circuits.